The present invention relates to a process for producing a silicon nitride sintered material showing no strength deterioration at high temperatures.
With respect to silicon nitride sintered materials produced from raw materials containing oxides of IIIa group elements including rare earth elements, for example, Japanese Patent Publication No. 7486/1973 discloses a process for producing a sintered material, which comprises mixing 85 mole % or more of silicon nitride (Si.sub.3 N.sub.4) with 15 mole % or less of at least one oxide selected from oxides of IIIa group elements, shaping the mixture, and sintering the shaped material in a non-oxidizing atmosphere. Also, Japanese Patent Publication No. 21091/1974 discloses a silicon nitride sintered material comprising 50% by weight or more of Si.sub.3 N.sub.4, 50% by weight or less of at least one oxide selected from Y.sub.2 O.sub.3 and oxides of La series elements and 0.01-20% by weight of Al.sub.2 O.sub.3.
In these silicon nitride sintered materials, however, there were problems that no sintered material having a high strength at elevated temperature could be obtained by mere additions of rare earth elements to silicon nitride and that addition having a Al.sub.2 O.sub.3 can give a high density but gives a grain boundary phase of low softening point and accordingly a significantly deteriorated high-temperature strength.
In order to solve the high-temperature strength problem, Japanese Patent Application Kokai (Laid-Open) No. 100067/1988 discloses a technique of adding, to a Si.sub.3 N.sub.4 powder, rare earth elements of given composition and given amount and firing the mixture to obtain a silicon nitride sintered material having a specific crystalline phase and accordingly a high strength at elevated temperature.
The silicon nitride sintered material disclosed in Japanese Patent Application Kokai (Laid-Open) No. 100067/1988 can achieve a high strength at elevated temperature to some extent, but the high-temperature strength is lower than the room-temperature strength. This is presumed to be because the grain boundary is crystallized but, with the above composition, a vitreous phase still remains slightly in the grain boundary. In order to reduce the amount of the vitreous phase remaining in the grain boundary, there is considered an approach of adding rare earth element oxides to silicon nitride in large amounts relative to the amount of the total oxygen (expressed as SiO.sub.2) present in the silicon nitride, to achieve a sintered material containing a vitreous phase in an amount as small as possible. In this approach, however, it is difficult to achieve a high density sintered material and.
Further, there was a problem that in firing a silicon nitride raw material in a casing, the amount of the shaped material to be filled into the casing is not specified and the properties of the sintered material obtained vary greatly depending upon the amount of the shaped material filled into the casing, even when the same firing conditions (e.g. temperature, time, pressure) are used.
The present inventors found that a silicon nitride sintered material showing no strength deterioration at high temperatures can be produced by using the technique disclosed in Japanese Patent Application Kokai (Laid-Open) No. 100067/1988 and by specifying firing conditions including the amount of shaped material to be filled into a casing. The finding has led to the completion of the present invention.